1. Field of the Invention
The present invention relates generally to automotive audio systems. More particularly, the present invention relates to a method and receiver capable of automatically matching output levels of different radio receiver input sources.
2. Discussion of the Related Art
In the automotive industry, the development of audio systems has generally depended upon the format of commercially available recordings and the feasibility of modifying standard audio systems for automotive use. A typical automotive audio system controls a stereo signal which is obtained from various different input sources such as an AM/FM antenna and tuners, a cassette tape player, or a compact disc player. A common problem with these different input sources relates to the electrical characteristics unique to each source. Differences in these characteristics substantially affect receiver signal processing and create an audio level matching problem currently inadequately addressed by present approaches to automotive audio systems. For example, a typical tape player will employ resistance divider networks in its audio path as part of internal signal processing and voltage division. These resistor divider networks either use fixed value components or require the manual adjustment of adjustable components. The values of the components are selected or are manually adjusted during the manufacturing process in an attempt to provide a known audio level at the output of the tape player. Each resistor in the network has a tolerance which can contribute to an overall tolerance stack within a given divider network. This creates a voltage attenuation that will be dependent upon a nearly infinite number of possible resistor combinations or on the accuracy and attentiveness of the technician making the manual adjustment. Thus, each tape player will have a unique audio output signal level which can only be approximated through statistical techniques and individual testing of every unit. This lack of predictability can result in a significant difference in the receiver""s audio output level when playing a cassette tape as opposed to listening to an FM radio signal, for example.
Present standards typically require a maximum of 3 dB difference in receiver audio output between any two input sources. In other words, an example standard would be given a 1 watt reference audio volume level, a specified AM input and a specified FM input must produce a receiver audio output level within 3 dB of one another. This receiver audio output tolerance must be held even though the individual AM and FM tuners might have been made by different manufacturers and may have different output levels relative to each other. Industry standard cassette tapes and CD""s are available for testing and design of tape and CD players with respect to these standards. Similarly, specifications are commonly known to describe modulation and Radio Frequency (RF) input settings for AM and FM signals. While all of these matching requirements are well documented and used within the industry, producing a radio receiver which will exhibit audio output levels which are relatively constant with the application of different input sources has proven to be difficult, if not impossible, under certain circumstances.
For example, another difficulty associated with the tape player issue is the inability to control voltage attenuation of the audio signal in reverse play mode and forward play mode separately. Since both modes share the same divider networks, the control for matching is typically shared as well. Producing receivers which use tape players as an audio input source and which will have the same audio output levels in both the forward and reverse tape modes is impossible if the same set of divider networks is used in both modes. In addition to the difficulty in meeting the 3 dB matching standard, the standard does not sufficiently address all types of sounds and music. It is therefore desirable to provide a system and method capable of automatically matching all types of radio receiver input sources with respect to output level. It is also desirable to match the audio output of the receiver with different types of audio input to tolerances of less than 3dB, but the present methods make this tightening of the matching standard unachievable on a consistent basis.
The present invention provides a computerized method for processing a radio receiver input signal generated from a selected input source. Processing results in the matching of output levels for a plurality of input sources. The method includes the step of creating alignment information for the plurality of input sources. The method further provides for storing the alignment information in a computer readable memory, and converting a required output level into a matched output level. The matched output level is based on the alignment information contained in the computer readable memory corresponding to the selected input source, and the required output level is obtained from a volume control setting.
The present invention also provides a method for aligning a radio receiver input source including the step of inputting a reference input signal from the input source to the radio receiver. The method further includes the step of setting a reference volume level on the radio receiver, wherein the reference volume level and the reference input signal determine a reference output level for the input source. The reference output level is then converted into an input source offset for the input source.
Additionally, the invention includes a radio receiver having a computer readable memory containing alignment information relating to a plurality of input sources. A volume control mechanism establishes a volume control setting for both the alignment and processing steps, and an audio processing system receives an input signal from a selected input source. The audio processing system further determines a required output level from the volume control setting, and converts the required output level into a matched output level based on the alignment information.
Further objects, feature and advantages of the invention will become apparent from a consideration of the following description and the appended claims when taken in connection with the accompanying drawings.